The present invention relates to compositions for preparing optical fiber coatings, optical fibers prepared with such coatings, and a method of modifying time-sensitive rheological properties of optical fiber coatings.
Optical fibers, including fiber optic ribbons, have acquired an increasingly important role in the field of communications, frequently replacing existing copper wires. This trend has had a significant impact in the local area networks (i.e., for fiber-to-home uses), which have seen a vast increase in the usage of optical fibers. Further increases in the use of optical fibers in local loop telephone and cable TV service are expected, as local fiber networks are established to deliver ever greater volumes of information in the form of data, audio, and video signals to residential and commercial users. In addition, use of optical fibers in home and commercial business for internal data, voice, and video communications has begun and is expected to increase.
Optical fibers typically contain a glass fiber (i.e., core and cladding) and at least two coatings, i.e., a primary coating and a secondary coating. The primary coating is applied directly to the glass fiber and, when cured, forms a soft, elastic, and compliant material which encapsulates the glass fiber. The primary coating serves as a buffer to cushion and protect the glass fiber core when the fiber is bent, cabled, or spooled. The secondary coating is applied over the primary coating and functions as a tough, protective outer layer that prevents damage to the glass fiber during processing and use.
Fiber optic ribbons contain a plurality of planar aligned optical fibers (i.e., coated optical fibers) that are circumscribed by a single or multi-layered coat of a matrix composition. Like the secondary coating of an individual optical fiber, the matrix functions as a tough, protective outer layer that prevents damage to the individual optical fibers during processing and use.
Certain characteristics are desirable for the primary coatings. The modulus of the primary coating must be sufficiently low to cushion and protect the fiber by readily relieving stresses on the fiber, which can induce microbending and, consequently, inefficient signal transmission. This cushioning effect must be maintained throughout the lifetime of the fiber. Because of differential thermal expansion properties between the primary and secondary coatings, the primary coating must also have a glass transition temperature (Tg) that is lower than the foreseeable lowest use temperature. The primary coating must also maintain adequate adhesion to the glass fiber during thermal and hydrolytic aging, yet be strippable therefrom for purposes of splicing or coupling. Poor adhesion can result in microbending and/or various sized delaminations, which can be significant sources of attenuation in the optical fiber.
Certain characteristics are desirable for the secondary coating on optical fibers and matrix coatings in fiber optic ribbons. Before curing, the coating composition should have a suitable viscosity and be capable of curing quickly to enable processing of the optical fiber or ribbon. After curing, secondary and matrix coatings should have the following characteristics: sufficient stiffness to protect the encapsulated glass fiber yet enough flexibility for handling (i.e., modulus), low water absorption, low tackiness to enable handling of the optical fiber or ribbon, chemical resistance, and sufficient adhesion to the underlying coating.
The ability of these different coatings to behave in a desired manner requires that each coating possess certain rheological properties. Typically, rheological properties of a particular material are considered with respect to their temperature sensitivity (i.e., temperature versus modulus curve), with the material possessing glassy attributes below the glass transition temperature and possessing rubbery attributes above the glass transition temperature. However, the rheological properties are also dependent upon the time (or rate) over which a stress is applied to the material.
Coatings are often exposed to different stresses during the lifetime of an optical fiber or fiber optic ribbon. It is desirable, therefore, for such coatings to perform in a predictable manner under each of these different stresses. For example, the coatings experience a low rate of stress as they are spooled and unspooled or handled in coupling the optical fiber to another optical fiber or light source. If the coating cannot accommodate a slow-rate stress, then the stress applied to the optical fiber or fiber optic ribbon can be transmitted to the underlying glass fiber(s). As a result, microbending of the glass fiber(s) may occur. In contrast to this slow-rate stress, the coatings experience a high rate of stress as they are stripped from the underlying waveguide core and during processing of the optical fibers. During stripping operations, a conventional stripping device (i.e., manufactured by Fujikura or Sumitomo) cleaves the coatings about a circumference of the optical fiber prior to stripping the coatings off the end of the glass fiber(s). During processing, primary coatings are often exposed to high-rate stress as coated fibers are drawn through a coating die during application of secondary or matrix coatings. Tearing of the primary coating can result in coating defects along the length of the optical fiber or ribbon.
Because of the different stresses placed upon the various coatings employed in optical fibers and ribbons, it would be desirable to provide compositions that can be used to prepare coatings having desirable rheological properties under both low-rate and high-rate stresses. The present invention overcomes this deficiency in the prior art.
One aspect of the present invention relates to a composition for preparing optical fiber coatings. The composition contains a polymerizable base composition, including at least one monomer and optionally at least one oligomer, wherein the polymerizable base composition is substantially free of unsaturated epoxidized diene polymers, and a tackifier present in an amount effective to modify a time-sensitive rheological property of a polymerization product of the composition. Also disclosed are optical fibers, fiber optic ribbons, and fiber bundles that contain a coating which is the polymerization product of a composition of the present invention.
Another aspect of the present invention relates to a method of modifying a time-sensitive rheological property of an optical fiber coating. The method includes introducing into a polymerizable composition a tackifier in an amount effective to modify a time-sensitive rheological property of the polymerization product of the polymerizable composition.
A further aspect of the present invention relates to a method of improving the strippability of one or more coating materials from an optical fiber. This method includes preparing an optical fiber comprising a fiber encapsulated by a coating, the coating being a polymerization product of a polymerizable composition comprising an amount of a tackifier effective to improve the strippability of the coating from the fiber core.
Another aspect of the present invention relates to a method of improving the processing characteristics of a coating material during preparation of an optical fiber. This method includes preparing an optical fiber comprising a fiber encapsulated by a coating, the coating being a polymerization product of a polymerizable composition comprising an amount of a tackifier effective to reduce the occurrence of coating failure during said preparing an optical fiber.
Still another aspect of the present invention relates to a method of reducing the occurrence of microbending during handling or use of an optical fiber. This method includes preparing an optical fiber comprising a fiber encapsulated by a coating, the coating being a polymerization product of a polymerizable composition comprising an amount of a tackifier effective to reduce the occurrence of microbending.
Applicants have discovered that introduction of a tackifier into a polymerizable composition can modify time-sensitive rheological properties of cured optical fiber coatings (i.e., a polymerization product) prepared from the composition. By modifying the time-sensitive rheological properties of the cured coatings, it is possible to prepare optical fiber coatings which will behave in a desired manner under certain stress situations, in particular high or low stress rates. Thus, optical fiber coatings will behave more like a glassy product when exposed to high stress rates and more like a rubbery product when exposed to low stress rates. This enables the coatings of the present invention to reduce the occurrence of microbending, to enhance the strip cleanliness of fibers during stripping operations, and to minimize the occurrence of processing defects during fiber or ribbon preparation.